Criticality in Two-Dimensional Quantum Antiferromagnets (2D QAF)

Low Dimensional (LoD) physics has long provided opportunities (both analytical and experimental) for exploring criticality in model systems that are more tractable than the more formidable 3D analogs. In particular, magnetic LoD compounds have provided multiple physical realizations with which theoretical predictions can be tested. The quantum (S = ½) two-dimensional model clearly reveals the influence of exchange anisotropy (Ising, XY, Heisenberg) and quantum fluctuations upon criticality: the Ising model spontaneously orders, the XY system undergoes a topological transition, and the Heisenberg analog (2D QHAF) remains disordered at any finite temperature. There are many crystalline compounds accurately described as 2D quantum Ising antiferromagnets but the equivalent XY and Heisenberg materials remain in development.
In practice, all known crystalline realizations of 2D QHAF magnetically order at low temperatures due to a combination of interlayer exchange (J_3d) and/or exchange anisotropy (Δ). Simulations of the ideal (J_3d=0) 2D QHAF [1] revealed that a ratio Δ/J < 0.01 will induce either XY or I-order at low temperatures and also revealed that the ideal 2D QHAF becomes a perfect 2D QXYAF in an applied field [2]. This talk will describe the approaches followed in the Clark University group in pursuit of the ideal 2D QHAF, as illustrated in the compound [Cu(pz)₂(2-HO-py)₂](PF₆)₂.
[1] Cuccoli, A. et al, Phys. Rev. B, 67, 104414 (2003).
[2] Cuccoli, A. et al, Phys. Rev. B, 68, 060402(R) (2003)